The scalar boson

The ATLAS experiment has released results confirming that the Higgs boson has spin 0 (it is a so-called “scalar”) and positive parity as predicted by the Standard Model, making it the only elementary scalar particle to be observed in nature.

Definitions of angular observables sensitive to the spin and parity of the decaying resonance in H→ ZZ*→ 4ℓ decay. (Image: CERN)

The Higgs boson, discovered by the ATLAS and CMS experiments in 2012, with a mass around 125 GeV, has had its properties thoroughly analysed by the ATLAS collaboration using the full data collected during the 2011 and 2012 first Run of the LHC. ATLAS has released updated results of studies of the Higgs boson spin (J) and parity (P), corresponding to the quantum numbers predicted by the Standard Model to be JP = 0+. The results exclude all alternative hypotheses of JP that were tested by ATLAS, in favour of the Standard Model prediction at more than 99% confidence level.

Spin is the intrinsic angular momentum of an elementary particle, measured in units of the reduced Planck’s constant ħ. In quantum field theory, the spin of a particle is related to its behaviour, for example particles with integer spin (0, 1, 2…), called bosons, can occupy the same quantum state at the same time. In contrast, particles with half-integer spin (1/2, 3/2, 5/2…) cannot. The known elementary constituents of matter (electron, quarks, neutrinos…) are spin 1/2 particles, whereas the particles (photon, W/Z, gluon) which mediate the known interactions (respectively electromagnetic, weak, strong) are spin 1 particles.

ATLAS has released updated results of studies of the Higgs boson spin (J) and parity (P), corresponding to the quantum numbers predicted by the Standard Model to be JP = 0+. The results exclude all alternative hypotheses of JP that were tested by ATLAS, in favour of the Standard Model prediction at more than 99% confidence level.

Parity is another intrinsic quantum property of a particle, a number P equal to +1 or -1, related to the particle properties under a reflection in space, for example to whether the decays of the particle into two mirror-symmetric configurations, have an identical probability or not.

The Higgs boson is predicted by the Standard Model to be a scalar particle (spin 0) of parity “+”, a state noted as JP = 0+.

The results are an update of previous spin and parity measurements by ATLAS (link 1), taking advantage of improved modeling and analysis strategies. The measurements are based on the H-> gg, H->ZZ* -> 4l and H->WW*->lvlv decay channels and their combination.

Owing to the tiny lifetime of the Higgs boson only its decay products are measured in the detector. The spin and parity of an unstable particle can be deduced by studying the detailed configuration of each decay event. For example in the H->gg decay channel, which has a simple configuration with only two photons in the final state, Figure 1 shows simulated distributions for two observables for the hypothesis that the decaying particle is the Standard Model Higgs boson (shaded histogram), along with hypotheses of various types of spin-2 particles (coloured lines). The first observable (pT) is the total transverse momentum of the photon pair, the second one (cosΘ*) is related to the direction of the photons seen in the center-of mass frame of the decaying particle. Discrimination between the hypotheses is seen for large pT and large cosΘ*.

In the H-> ZZ* -> 4 leptons channel, the direction and momentum of the well-identified leptons contain rich information about the spin and parity of the decaying particle (Figure 2).
Combining the measurements in the H-> gg, H-> ZZ* and H-> WW* channels, the description of the data in terms of a hypothetical spin-2 particle or 0- particle (instead of the Standard Model Higgs boson) is excluded at more than 99% confidence level.
In addition, the H -> ZZ* and H -> WW* channels, with the information embedded in the final state particles, allow one to set limits on a possible admixture of a “0-“ state along with the standard 0+ Higgs Boson.

Expected distributions of pTγγ (left) and |cos θ*| (right), for a SM Higgs boson (shaded histograms) and for spin-2 particles, for three different choices of the QCD couplings. (Image: CERN)